Applicants claim priority under 35 U.S.C. xc2xa7119 of German Application No. 198 40 224.4 filed Sep. 3, 1998. Applicants also claim priority under 35 U.S.C. xc2xa7120 of PCT/EP99/06274 filed Aug. 26, 1999. The international application under PCT article 21(2) was not published in English.
1) Field of the Invention
The invention relates to a process for preparing catalysts for the partial gas-phase oxidation of saturated and/or unsaturated C4-hydrocarbons to form maleic anhydride (MA).
2) Background Art
The preparation of maleic anhydride from C4-hydrocarbons has been known for more than 20 years. It is carried out using catalysts based on vanadyl phosphates (vanadyl pyrophosphate, vanadium-phosphorous oxides). These vanadyl phosphates are prepared via a catalyst precursor which can be prepared in aqueous or organic medium. The precursor is then, in a second step before or after shaping, converted either in the reactor (in situ) or externally into the actual catalytic reactive substance.
The reaction of the C4-hydrocarbon over the catalyst is carried out in various types of reactor. Reactors used are fixed-bed, fluidized-bed and also riser reactors; in the case of fixed-bed reactors, use is made exclusively of catalysts based on vanadyl phosphates in the form of unsupported catalysts. EP-B 72381 and WO-A 96/25230 disclose the use of coated catalysts too.
The precursor can be prepared either in an aqueous medium or in an organic medium. In industry, preference is given to carrying out the preparation in an organic solvent. Here, isobutanol (2-methylpropan-1-ol) has proven useful as solvent or reaction medium. Compared with the catalysts prepared in an organic medium, those prepared in an aqueous medium have significantly lower activity and selectivity. A cause of the difference is the lower specific surface area of the catalysts prepared by the aqueous route. Furthermore, the use of an organic solvent brings process engineering advantages, namely the precursor formed is easier to separate from the reaction mixture since the product is insoluble in the solvent. The solvent has to meet a number of prerequisites. Thus, for example, it must not react with the phosphorous compounds required for formation of the vanadyl phosphate. In addition, its boiling point has to be within a range in which the formation of the desired vanadyl phosphates is observed. As is generally known, the reaction temperature strongly influences the rate of catalyst formation. Since the reaction is usually carried out in the boiling solvent, the boiling point of the solvent plays an important role. The solvent also has a strong influence on the morphology of the reaction product and sometimes even on the phases formed (Doi, T.; Miyake, T.; Applied Catal. A. General 164 (1997), 141-148).
The vanadium-containing starting compound used is generally vanadium pentoxide. Phosphoric acid generally serves as phosphorous compound. Since the vanadium is present in the oxidation state IV in the target compound, a reduction by means of a suitable reducing agent is necessary. Many compounds have been described for this purpose. Thus, use is made of, for example, gaseous HCl, oxalic acid, benzyl alcohol, isobutanol, hydrazine and also many other reducing agents. The nature of the reducing agent plays a minor role in the reaction.
U.S. Pat. No. 4,132,670 describes a process for preparing vanadyl phosphate catalysts. In this process, vanadium pentoxide is reacted in an alcohol, preferably isobutanol, to give a vanadium(IV) compound. The reaction of vanadium(V) to vanadium(IV) is effected by the solvent used. Subsequently, the vanadium phosphate precursor is prepared using concentrated H3PO4. The water content of the reaction mixture in the preparation of the precursor is here described as an important parameter and a low water content in the reaction mixture is recommended.
U.S. Pat. No. 4,382,876 discloses a process for preparing catalysts in which a vanadium(V) compound and isobutanol as solvent are placed in a reaction vessel and H3PO3 as reducing agent is added in relatively small amounts. The reduction of the vanadium(V) compound to vanadium(IV) in that invention occurs by means of a combined reduction by the tertiary alcohol and the phosphorous acid. The water formed in this process together with the solvent are distilled off batchwise a number of times. This procedure has the disadvantage that further water is continuously formed in the reaction mixture and this is only distilled off after a certain time. Thus, a continually low water content is not possible, which leads to a poorer performance of the future catalyst.
WO-A 95/29006 likewise discloses a process for preparing vanadyl phosphate catalysts. Here too, a vanadium(V) compound is reacted with phosphoric acid in isobutanol. In. this process, the water content is controlled by addition of highly concentrated phosphoric acid to the reaction mixture in order to absorb the water of reaction formed. The total water content of the reaction mixture is here determined by the phosphorous compounds used (85% H3PO4, 100% H3PO4, 106% H3PO4 or polyphosphoric acids) and by the water formed in the reaction (by the reduction of vanadium(V) to vanadium(IV) compounds). Major-disadvantages of this process are the high costs and the difficulty of handling concentrated phosphoric acid and polyphosphoric acid.
The reaction of the starting substances to form the desired vanadyl phosphate can in principle be carried out in various ways:
1. reduction of the vanadium(V) compound to vanadium(IV), subsequent reaction with the phosphorous compound to give vanadyl(IV) phosphate,
2. reaction of the vanadium(V) compound with the phosphorous compound to form the corresponding vanadium(V) phosphate and subsequent reduction to vanadyl(IV) phosphate, or
3. parallel reduction and reaction with the phosphorous compound to give vanadyl(IV) phosphate.
After the reaction is complete, the suspended or dissolved vanadyl phosphate is separated from the reaction mixture by suitable methods, for example filtration or evaporation, dried and converted into the active catalyst in a further step, namely calcination or activation. Shaping for use in fixed-bed reactors is carried out either before or after calcination, for example by tableting or by extrusion. A further possible way of shaping the catalyst is the production of coated catalysts as are also used in other processes, for example the synthesis of phthalic anhydride from o-xylene or naphthalene.
The calcination of the precursor, i.e. the conversion into the actual catalyst, can be carried out either externally or in the reactor (in situ) in which the conversion of the C4-hydrocarbons into maleic anhydride is carried out.
U.S. Pat. No. 4,382,876 discloses a process in which the precursor obtained in the first step is converted into the actual catalyst in a second step in the reactor itself. Here, the precursor is activated by heating at a rate of less than 1xc2x0 C./min in order to avoid hot spots in the catalyst. This in-situ calcination is carried out in the presence of C4-hydrocarbon and air and the precursor is heated to a temperature of from 450xc2x0 C. to 510xc2x0 C. After this temperature has been held for from about 12 to 72 hours, the calcined material is slowly cooled again.
A major disadvantage of this process is that the final activity of the catalyst is reached only after some hundreds of hours.
Apart from the calcination processes described, external activations are also known. WO-A 95/29006 describes a process for calcination in which the precursor is reacted in an atmosphere of air, water vapour and inert gas. Here, the precursor is slowly heated at a heating rate of less than 1xc2x0 C./min in a number of steps to a temperature in the range from 350xc2x0 C. to 550xc2x0 C. This temperature is held for from 2 to 8 hours, giving a mean oxidation state of vanadium of less than +4.5. U.S. Pat. No. 5,185,455 discloses a similar process for external calcination. Here, the tableted precursor is heated stepwise in a defined gas atmosphere to a temperature of 425xc2x0 C. (first temperature ramp up to 250xc2x0 C.) and held at this temperature for a period of 6 hours. The oxygen and water content of the calcination gas plays an important role in determining the future performance of the catalyst.
These processes have economic advantages since they do not occupy the reactor for a number of weeks at reduced yield until the catalyst has been completely activated, as is the case in in-situ calcination. In addition, the precursor can be activated in relatively small sub-batches in the external calcination, which allows continuous quality monitoring and the entire catalyst batch is therefore not subjected to the risk of loss. The external calcination enables the conditions required for optimum activation of the precursor to form the active catalyst, (e.g. oxygen content and temperature) to be set more uniformly and reliably. In contrast, the in-situ calcination in a fixed-bed reactor leads, for example, to nonuniform conversion of the catalyst across the reactor or along the length of the reactor tube.
The external calcination of the precursor is generally carried out in defined gas atmospheres (gas mixtures of oxygen, inert gas and water vapour) and using a precisely defined temperature programme. The catalyst formed using this procedure immediately has its full activity, in contrast to the catalyst calcined in situ. The oxygen content (control of the mean oxidation state of vanadium), the concentration of water in the furnace atmosphere and the parameters of the temperature programme (steepness of the temperature ramps, final temperatures and hold times) are important in influencing the performance of the catalysts obtained.
To control the catalytic properties of the catalysts, U.S. Pat. No. 4,132,670, EP-A 0458541, DE-A 3018849 (U.S. Pat. No. 4,251,390), WO-A 93/01155, U.S. Pat. No. 5,288,880 have proposed the addition of many compounds as promoters or dopants in widely differing concentrations. Compounds used here are, for example, compounds of the elements Mo, Zn, Fe, Co, Li, Ce, Zr, U, Bi and Cr, with atom number ratios of vanadium:promoter element in the range from 1:0.2 to 1:0.001 being described. The promoter can be added at various points in time during the production process. Use has been made of techniques in which the addition is carried out during the reaction to form the precursor (homogeneous distribution of the promoter in the precursor) or else impregnation methods by means of which the previously synthesized precursor is surface-coated with the promoter.
To control the structure of the catalyst formed, EP-A 0151912 describes the use of surface-active, alcohol-modifying substances such as HI, SO2 or fuming sulphuric acid.
Apart from the use of promoters, the mixing-in of inert materials to control the catalyst activity has also been described. Examples of inert additives are SiO2, TiO2 and silicon carbide. These materials are generally added after the synthesis of the precursor is complete and before the catalyst is shaped.
Furthermore, the use of additives which control the pore structure of the shaped catalyst used have also been described relatively recently. Here, for example, certain amounts of higher alkanoic acids are added to the dried precursor before shaping. In an additional process step, these substances are finally removed again from the shaped body after shaping, as a result of which a defined pore structure can be achieved.
The processes which have been described for preparing catalysts for the synthesis of maleic anhydride all have a number of disadvantages. Thus, for example, the reduction of vanadium(V) by means of hydrogen chloride gas results in increased corrosion within the plants. The resulting waste chlorine gases and chloride wastes have to be disposed of at considerable expense and sometimes contaminate the precursor. A further disadvantage is associated with the amounts of water formed as a result of the reaction. The methods described in the prior art for removing water from the reaction mixture either impair the performance of the future catalyst or can only be carried out with difficulty and at high cost, for example when using polyphosphoric acid.
It is therefore an object of the invention to develop a process for preparing catalysts for the synthesis of maleic anhydride by gas-phase oxidation, which process avoids the abovementioned disadvantages.
The invention provides a process for preparing catalysts based on vanadyl phosphates for the synthesis of maleic anhydride by oxidation of saturated and/or unsaturated C4-hydrocarbons by reacting a vanadium(V) compound with a mixture of phosphorous and phosphoric acids in a solvent mixture, characterized in that the molar H3PO3/H3PO4 ratio is from 1:1 to 1:2.5, the solvent used is a mixture of isobutanol, a structure former selected from the group consisting of mono-functional and polyfunctional alcohols, monofunctional and polyfunctional amines, organic phosphates, phosphites and phosphonates, and an entrainer selected from the group consisting of alkylaromatics and cycloalkanes, where the molar ratio of vanadium to structure former is in the range from 10:1 to 100:1, and the water of reaction together with entrainer is continuously removed at the boiling point, where the residual water content of the runback and the resulting residual water content of the reaction mixture is less than 0.5% by volume, and the precursor obtained in this way is subsequently subjected to a calcination in which a temperature programme comprising 3 ramps is employed, where the ramps are as follows:
ramp 1: initial temperature: xe2x89xa650xc2x0 C., heating rate: from 5 to 20xc2x0 C./min, final temperature: from 100 to 250xc2x0 C., hold time: from 0 to 3 h,
ramp 2: initial temperature=final temperature of ramp 1, heating rate: from 1 to 10xc2x0 C./min, final temperature: from 150 to 300xc2x0 C., hold time: from 0 to 3 h,
ramp 3: initial temperature=final temperature of ramp 2, heating rate: from 0.1 to 3xc2x0 C./min, final temperature: from 380 to 460xc2x0 C., hold time: from 2 to 8 h,
and gas mixtures of air, inert gas and water vapour in a volume ratio of 0.1-0.5:0.1-0.5:0.0-0.8 are used during these temperature ramps.
A vanadium(V) compound is suspended in a mixture of an agent which influences the structure of the vanadyl phosphate, for example benzyl alcohol or isobutanol, and an auxiliary which makes it possible to remove the water formed and/or introduced (known as an xe2x80x9centrainerxe2x80x9d). The suspension is heated and, after reaching the boiling point, a mixture of phosphorous acid and phosphoric acid is added. As vanadium(V) compound, preference is given to using vanadium pentoxide. Other vanadium(V) compounds, for example its oxychlorides, can also be used.
As phosphorus components, use is made of phosphorous acid and phosphoric acid. The phosphorous acid serves both as reducing agent for the vanadium(V) and as a source of phosphorus. The use of a further reducing agent can therefore be dispensed with. The phosphorous acid is preferably used in solid form for preparing the mixture, but can also be used as a solution. Preference is given to a phosphorous acid having a purity/strength of from 85 to 100% by weight, particularly preferably 85% by weight. The phosphoric acid mixture is prepared by dissolving the phosphorous acid in the phosphoric acid, if appropriate with gentle heating. The mixture of H3PO3/H3PO4 has a molar mixing ratio of from 1:1 to 1:2.5, preferably from 1:1.5 to 1:2.0.
The vanadium compound and the phosphorus compounds are used in such amounts that the atom number ratio of vanadium:phosphorus is in the range from 1:0.9 to 1:2.0. The vanadium-phosphorus compound obtained in this way has a mean vanadium oxidation state of from 3.85 to 4.3, with the molar vanadium: phosphorus ratio in the isolated compound being in the range from 1:0.9 to 1:1.1.
The mixture of phosphorous acid and phosphoric acid is added immediately after the boiling point of the vanadium pentoxide/solvent suspension has been reached. A xe2x80x9cprereduction periodxe2x80x9d as is described in other patents is not employed.
After the addition of the phosphorus component is complete, the reaction mixture is held under reflux conditions for a period of from 4 to 24 hours, preferably from 12 to 20 hours.
The solvent in which the reaction is carried out is isobutanol (2-methylpropan-1-ol). However, it is also possible to employ other inert solvents having a boiling point of about 100xc2x0 C., for example toluene. An important aspect of the process claimed is that only a small amount of a structure-modifying agent has to be added.
As structure modifier, it is possible to use the following classes of compound: monofunctional and polyfunctional alcohols, for example ethanediol, 1,3-propanediol, n-hexanol, diphenylmethanol, preferably benzyl alcohol and alpha-omega-alkanediols, monofunctional and polyfunctional amines, for example n-propylamine, isobutylamine, ethylenediamine, preferably n-decylamine and propylenediamine, organic phosphates, for example butyl phosphate, dibutyl phosphate, trimethyl phosphate, preferably tributyl phosphate, phosphites or phosphonates, for example methylphosphonic acid, trimethyl phosphite, dibutylphosphonic acid, preferably tributyl phosphite, or mixtures thereof. owing to their chemical structure, these compounds can be incorporated (intercalated) in the layer structure of the precursor and thus influence the morphology of the material, i.e. the type and number of defects formed. The molar ratio of structure modifier to vanadium used is from 0.1% to 10%. The small amount required clearly distinguishes the process described here from other methods of preparation in which the additions, in particular the addition of benzyl alcohol, are made in a molar ratio of vanadium : additive of from 1:0.5 to 1:1.0. These added substances there serve as reducing agent; a structure-modifying effect when using small amount is not described (EP-A 0151912).
In addition, xe2x80x9centrainersxe2x80x9d are added to the reaction mixture. These make it possible to remove water formed during the reaction from the reaction mixture. A number of substances can be used as entrainers. In general, use is made of nonpolar compounds which are only slightly miscible with water, for example alkylaromatics and cycloalkanes. Owing to their ready availability, preference is given to using toluene, xylenes, cyclohexane and benzene. Cyclohexane is particularly preferred as entrainer. The amount added to the isobutanol lies in a range from 5 to 15% by volume, preferably from 8 to 12% by volume, of the amount of solvent used.
While the reaction mixture is boiling, the ternary mixture of solvent, entrainer and water of reaction formed is continuously taken from the reactor via an external circuit. Within this circuit, the ternary azeotrope is conveyed into a vessel which allows the separation of the 2-phase condensate and thus the removal of the water-rich phase. The mixture of solvent and entrainer which has been depleted in water is recirculated to the reactor. After the separation, the residual water content of the recirculated mixture is less than 0.5% by volume. This procedure ensures that the water content of the reaction mixture is always kept below 0.5% by volume. This is an important criterion in respect of the selectivity and conversion of the finished catalyst.
In the preparation according to the invention of the precursor, doping of the catalyst may, if desired, also be carried out. Doping of the catalyst can be carried out in various ways. One method is to add metal compounds in the form of soluble compounds of the elements Mo, Wo, Bi, Cr, Co, Ni, Fe, Li, Ce, Zr, U and Zn, preferably Mo, Cr, Bi, Co, Zn, Li and Ce, to the organic solvent mixture in an atom number ratio of vanadium:doping element of from 1:0.1 to 1:0.001. A further way is to dope the catalyst precursor formed after the. reaction is complete and immediately before drying or separating off. Here too, soluble compounds have to be used in order to achieve a uniform distribution of the dopant. Examples of soluble compounds are the chlorides, carbonates, acetylacetonates, acetates and nitrates of the abovementioned metals.
After the refluxing time has expired, the precursor is separated off. This can be achieved by filtration and subsequent drying. It can be particularly advantageously done by spray drying of the suspension obtained.
After the precursor has been dried, it can be used directly for further processing to produce coated catalysts, catalysts for processes in fluidized catalyst beds or unsupported catalysts for fixed-bed reactors. For this purpose, it is, however, necessary to bring the precursor into a form which is advantageous for the respective application. This shaping can be carried out, for example, by tableting, agglomeration, extrusion or a similar process. The nature of the shaping process and the resulting catalyst shape and nature has no influence on the applicability of the process. For the shaping step, auxiliaries which, for example, influence the shapability and flowability of the raw material can be added. Auxiliaries which may be used are, for example, graphite, higher alkanoic acids (e.g. stearic acid), polyethylene glycols and silicas, or mixtures thereof. Possible shapes of the catalyst bodies are, for example, cylinders, spheres and rings.
The precursor is subsequently, regardless of the form into which it has been shaped, subjected to a calcination process in which it is converted into the actual catalytically active form. The calcination can be carried out either in the reactor (in situ) or in an external step. External calcination is to be preferred since it leads to catalysts which are fully ready-to-use immediately after the process. This makes a prolonged activation phase, as is observed in catalysts calcined in situ, unnecessary.
The calcination carried out in the context of the process described here is preferably carried out in the following manner using the dried precursor as powder or shaped body:
The precursor is introduced into a suitable furnace system, for example a fluidized-bed furnace in the case of powders or a tray or tube furnace in the case of shaped bodies, which is equipped with a unit for metering-in gases and for controlling the temperature over time. The parameters described below relate to the external calcination of unsupported catalysts:
1st Calcination Step: Making the Furnace Chamber Inert
In a first step, the furnace chamber is made inert at a temperature in the range from 20xc2x0 C. to 100xc2x0 C. using an inert gas. As inert gas, preference is given to using nitrogen. The time and amount of inert gas required for making the system inert should be such that a residual oxygen content in the furnace system of less than 5% by volume is ensured.
2nd Calcination Step: Ramp 1
In the next step, a gas mixture having an oxygen content of from 5% by volume to 20% by volume is introduced into the furnace system. The furnace temperature is then increased at a rate of from 5xc2x0 C./min to 20xc2x0 C./min (ramp 1) to a temperature of from 150xc2x0 C. to 250xc2x0 C. (final temperature 1). If desired, this temperature can be held for a period of up to 3 hours.
3rd Calcination Step: Ramp 2
After the final temperature 1 has been reached and the hold time has expired, the heating of the system is continued using a ramp 2 up to a final temperature 2 of from 200xc2x0 C. to 300xc2x0 C. The heating rate is from 1xc2x0 C./min to 10xc2x0 C./min and the gas mixture fed into the furnace system consists of the components oxygen, water vapour and inert gas, with an oxygen content of 5% by volume and a water content of 50% by volume being achieved. After the final temperature has been reached, a hold time of up to 3 hours can be inserted in this step too.
4th Calcination Step: Ramp 3
After the final temperature 2 has been reached and the hold time has expired, the heating of the system is continued using a ramp 3 up to a final temperature 3 of from 380xc2x0 C. to 460xc2x0 C. Here, a gas mixture having a composition corresponding to that for ramp 2 is used. The heating rate is in the range from 0.1xc2x0 C./min to 3xc2x0 C./min. This final temperature is held for a period of from 2 to 8 hours while maintaining the gas composition.
5th Calcination Step: Cooling Phase
After the 4th calcination step has been concluded, the furnace system is cooled to ambient temperature under inert gas. When the temperature has dropped below 100xc2x0 C., the furnace system can be opened and the catalyst can be taken out. The catalyst is stored in tightly closed containers.
The addition of water vapour during calcination is of great importance since it has a positive influence on the conversion of the precursor into the catalytically active vanadyl pyrophosphate. Samples which have not been treated with water vapour have a significantly lower catalyst performance. Air is preferred as oxygen-containing gas. The oxygen concentrations required for the individual calcination steps can advantageously be achieved by mixing air with an inert gas, preferably nitrogen. The gas mixture of air, inert gas and water vapour can be used in a volume mixing ratio of 0.1-0.5:0.1-0.5:0.0-0.8.
The calcination method employed here differs clearly from the prior art in terms of the temperature ramps used. To shorten the time of the calcination process, the furnace system is heated to a temperature of from 150xc2x0 C. to 250xc2x0 C. using a temperature ramp which is significantly steeper than in the texts cited for comparison. In the present application, a temperature ramp of from 1 to 10xc2x0 C./min is used. This heating rate, which is up to 10 times that of the prior art, has made it possible to provide a calcination process which has a time advantage over comparable processes and therefore operates more economically. There are additional differences in the gas composition in the individual calcination phases.